PROJECT SUMMARY Thiopurines such as mercaptopurine are S-substituted antimetabolites used as functional analogs to natural nucleobase precursors. They are highly effective against hematologic malignancies (leukemia and lymphoma) and autoimmune diseases (inflammatory bowel diseases [IBD]), with an estimated patient population >350,000 in the US. However, the cytotoxicity of thiopurines depends on their conversion into 6-thioguanine (TG) nucleotides (TGN), which are incorporated into DNA, causing cell death by post-replicative DNA mismatch repair. They have narrow therapeutic indexes with dose-limiting hematopoietic toxicity whereas low-responders are undertreated with standard dosing. Therefore, there is enormous clinical benefit from preemptively identifying patients at risk of thiopurine toxicity and individualizing therapy to mitigate it. We propose here a high throughput method based on matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) for quantifying the pharmacological endpoint of thiopurines, TG incorporated in chromosomal DNA (DNA-TG) of white blood cells, and examine its specificity and accuracy in standard and clinical samples. To test this strategy, we propose three specific aims: Aim #1. Chemical derivatization, isolation and detection of 6-thioguanine (6-TG) by MALDI-MS. Aim #2. Examination of linear range, sensitivity, specificity and reproducibility of the method. Aim #3. Validation of high throughput 6-TG detection with biological samples. With the aim to individualize thiopurine therapy through reducing adverse reactions and obtaining optimum therapeutic efficacy for patients on thiopurine treatment, the high throughput technique facilitates continuous monitoring of thiopurine in DNA before and during the therapy. This highly translational project will likely have high impact on a large number of patients who are under chemo-treatment or thiopurine therapy for non-malignant conditions (e.g., IBD).